Methodology

1. Overview

Every calculator on DoIt International is built from a documented, standard formula and a named data source. On each calculator page you will find a “How this is calculated” section that states the formula, a worked example, and the assumptions behind the result. This page summarises the common formulas, data sources, update cadence, and review process.

2. Finance calculators

Loan and mortgage payments use the standard amortisation formula P = L · r · (1+r)ⁿ / ((1+r)ⁿ − 1), where L is the loan amount, r is the periodic interest rate, and n is the number of periods. Compound interest uses A = P(1 + r/n)^(nt); simple interest uses A = P(1 + rt). Debt-to-income, ROI, net worth, VAT, percentage, tip, discount, and markup calculators use their standard arithmetic definitions.

Finance calculators do not call any external API. All inputs are entered by you and computed in your browser. Results are estimates for planning only and are not financial advice.

3. Health & fitness calculators

BMI is kg/m². BMR uses the Mifflin–St Jeor equation; TDEE multiplies BMR by an activity factor. Body fat uses the U.S. Navy circumference method. Heart-rate zones use percentages of maximum heart rate (220 minus age). One-rep max uses the Epley formula. BAC uses the Widmark equation. Water intake, macros, VO₂ max, BSA, and sleep-cycle calculators use their standard published formulas. Each page names the formula it applies.

Health calculators are for general information only and are not medical advice. Always consult a qualified health professional for decisions about your health.

4. Currency converter

Live exchange rates are fetched from the public open.er-api.com rates API (GET /currency?base=XXX). Rates are refreshed when you change the source currency or press refresh. The timestamp shown is the time the rates were last fetched. Rates are indicative only and may differ from the rate your bank or provider offers.

5. Unit converter

Length, volume, and weight conversions use internationally recognised conversion factors (for example, 1 inch = 25.4 mm, 1 lb = 0.45359237 kg, 1 gal (US) = 3.785411784 L). Temperature converts between Celsius, Fahrenheit, and Kelvin using the exact linear relationships. Each result shows the factor applied.

6. Money & travel and cooking & DIY calculators

Trip budgets, tipping, exchange-rate fees, remittance costs, cost-of-living comparisons, and similar money & travel calculators combine the formulas above with user-entered amounts and published reference rates. Cooking and DIY calculators (recipe scaling, oven temperatures, measurement conversion, paint, tile, wallpaper, concrete, and floor area) use standard culinary and construction conversion factors. Shoe and clothing size converters use published international size charts and default the target region to your UI language.

7. Translation, dictionary, and synonyms tools

The translator sends your text to the MyMemory machine-translation API. The dictionary looks up Wiktionary (with canonical-title resolution) and falls back to the Free Dictionary API for single English words. The synonyms finder uses the Datamuse word-association API. Source language can be detected automatically or set manually. These services are third-party and their results may contain errors.

8. Update cadence

Exchange rates update on each request to the rates API. Calculator formulas and conversion factors are reviewed at least annually, and whenever the underlying standard, regulation, or data source changes. Policies are dated and re-reviewed at least annually. When a calculator or guide is updated, the change is logged in our internal review record.

9. Review process

Before publication, every calculator is checked by a second team member: the formula is verified against an independent source, the worked example is recomputed by hand, and the copy is read for accuracy and plain language. Guides and landing pages receive the same editorial review. Errors reported by readers are triaged within two business days and corrected; see our Editorial Policy for how corrections work.

10. Limitations

Our tools provide estimates and general information, not professional advice. Exchange rates are indicative. Machine translation and dictionary results may contain errors. Always confirm important results with a qualified professional or your service provider. See our Disclaimer.